A vehicle hydraulic brake system of the brake-by-wire type is disclosed e.g. in the below-identified Patent document 1. In this brake system, hydraulic pressure supplied from a high hydraulic pressure source is adjusted by a pressure regulator to a value corresponding to brake operation by the driver, or to a value set independently of brake operation by the driver, by an electronic control unit, and brake hydraulic pressure is generated by actuating a master piston of master cylinder using the hydraulic pressure adjusted by the pressure regulator as assist pressure.
The brake system disclosed in FIG. 3 of Patent document 1 includes an input piston configured to be advanced under the brake operating force, a pressure regulator (pressure regulating means) which adjusts hydraulic pressure supplied from a high hydraulic pressure source to a value necessary to generate a target braking force and feed the thus adjusted hydraulic pressure into a third hydraulic pressure chamber (boost chamber), and a master cylinder configured to generate brake hydraulic pressure by driving a master piston (intermediate piston) under the hydraulic pressure adjusted by the pressure regulator.
The input piston is axially slidably inserted in a hole formed in the rear end of the master piston such that the front end of the input piston protrudes into a relief chamber defined in the master piston. The rear end of the master piston protrudes into the third fluid chamber. The gap between the relief chamber of the master piston and the third fluid chamber is sealed by a high-pressure seal disposed between the inner peripheral surface of the bore of the master piston and the outer periphery of the input piston.